1. Field of the Invention
This invention relates to the mechanical arts. In particular, it relates to instruments for measuring and controlling the flow of fluids, such as gases.
2. Discussion of Relevant Art
There have been developed in the art a variety of instruments for measuring and controlling the mass flow of gases ranging from below 5 standard cubic centimeters per minute (SCCM) to more than 500,000 SCCM. The prevalent design of such instruments requires that the flow of the gas be divided into two or more paths.
In a typical flow meter, a small amount of gas is routed through a flow sensor assembly, where the mass flow is measured, while most of the flow is routed through a splitter section located in parallel with the flow sensor assembly. When a flow meter is operationally connected with a valve, the flow meter can be used to control, as well as measure, the flow of gases.
The flow sensor assembly contains a flow tube that carries two resistance thermometers on its outside surface. The resistance thermometers form two legs of an electronic bridge circuit; the other two legs are fixed resistors. The flow sensor assembly must be carefully manufactured, so that the two resistance thermometers are as identical as possible in electrical and thermal characteristics.
When a voltage is applied across the bridge circuit, current passes through the resistance thermometers causing them to self-heat. Since they are nearly identical in electrical and thermal characteristics, the temperature of each resistance thermometer increases by the same amount, causing the electrical resistance in each to increase by the same amount--for so long as there is no gas flow through the flow tube. As soon as gas flow occurs, the upstream resistance thermometer is cooled by heat transfer to the flowing gas and the downstream resistance thermometer is either heated or cooled to a lesser extent. The average temperature of the upstream resistance thermometer will now be different than the average temperature of the downstream resistance thermometer and the bridge circuit will show an imbalance due to the differences in their resistance.
During manufacture, mass flow meters are calibrated to produce a specific output for a given flow of a specific gas. However, over a period of time, the mass flow meters become uncalibrated and their output changes. This phenomenon, called calibration drift, results in inaccurate measurement and control of the gas flow. Calibration drift can occur if the properties of the flow meter's thermal or electrical systems change from the time of calibration. Common sources of calibration drift include changing thermal conductivity of foams, plastics, wire insulations and epoxies used in fabricating the mass flow meter as they outgas and age; changing resistance of the resistance thermometers as residual stresses are relieved due to repeated heating and cooling of the flow sensor assembly; random minor changes in the geometry of the flow sensor assembly changing the heat transfer to and from the flow sensor assembly; drift in the electronics that amplify and deliver the sensor signal; physical damage to the calibration section; and a buildup of contaminants inside the flow tube impeding heat transfer to the gas and adding additional mass to be heated or cooled during transient flow conditions.
The measurement and control of the flow of gases is important in many industries. During the manufacture of semiconductors, for example, many processes require a precise reaction of two or more gases under carefully controlled conditions. The extreme precision required to make solid state memories, where millions of transistors are deposited on an area the size of a fingernail, could not be possible without the accurate control of the process gases.
As the size of individual devices in integrated circuits has shrunk from several microns to less than one, and as the number of devices per circuit has increased from a few thousand to several million, the accuracy of control of the equipment used to manufacture the devices has become increasingly important. If a flow meter is uncalibrated, the process gases are not supplied in the proper amounts and the resulting integrated circuits exhibit degraded performance.
It is not always obvious which step in the complex manufacture of semiconductors is responsible for this degraded performance. Consequently, there is a need for a reliable and cost effective apparatus and method for in-line calibration verification of mass flow meters.
Presently, semiconductor manufacturers use four techniques to ensure the accuracy of mass flow meters:
1. Inspecting and testing the silicon wafers after their manufacture has been completed;
2. Attaching a reference meter to the end of a gas-line containing an in-line flow meter, flowing gas through the in-line and reference flow meters and then comparing the output of the reference meter to the output of the in-line flow meter;
3. Removing the flow meter from the gas-line and replacing it with a freshly calibrated flow meter; or
4. If the manufacturing process is conducted in a vacuum chamber, sealing the chamber discharge and measuring the rise in the pressure and temperature of the known volume of the chamber, after a specific time, during which the flow rate of a gas into the chamber remains constant. The flow rate calculated for the known conditions can then be compared to the flow rate indicated by the flow meter.
The second technique involves establishing a known flow of gas through the flow meter, as a reference flow, and observing the flow meter's output. This technique suffers from several drawbacks. Access to the fluid flow system is often limited. In addition, the process of attaching and subsequently removing the reference meter can cause problems with the purity of the reactant gases.
The extreme accuracy required in the fabrication of solid state devices has led some manufacturers to adopt the third technique to ensure that there has been no change in the calibration of their flow meters. Based on the possibility that the flow meter may have become uncalibrated, this technique requires the scheduling of mass flow meters for periodic removal and shipment back to the manufacturer for recalibration.
Typically, the removal of a mass flow meter from a high purity gas line will require hours or days of purging the gas line and the associated equipment to return the system to an acceptable level of purity, before the flow meter and associated equipment can be put back into service. The cost of shutting down the equipment can dwarf the more obvious cost and effort associated with the shipment and recalibration of the mass flow meter itself.